The global burden of stroke is substantial and growing, yet our ability to treat acute ischemic stroke is severely limited. Although considerable effort has been made in the past 2 decades, neuroprotective agents for treatment of acute stroke have largely failed in clinical trials. The success of thrombolysis and endovascular treatment demonstrates that rapid reperfusion of the ischemic brain is an effective means to prevent injury; however, these therapies are currently limited by a short time window for which it provides benefit and low rates of reperfusion. The long-term goal of this project is to understand how post-ischemic reperfusion affects brain parenchymal arterioles (PAs) - high resistance pre-capillary vessels - in ways that would limit blood flow to the ischemic region, increase perfusion deficit, and promote expansion of infarct. Our central hypothesis is that ischemia and reperfusion (I/R) cause excessive vasoconstriction of PAs that promotes incomplete reperfusion and restriction of capillary flow. We further hypothesize that chronic hypertension, a common co- morbid condition of stroke patients, increases PA vasoconstriction and impairs vasodilation through endothelial dysfunction, restricting reperfusion and worsening stroke outcome. These hypotheses are based on our preliminary studies that demonstrate that unlike middle cerebral arteries that undergo prolonged vasodilation in response to I/R, PAs have enhanced tone due to calcium sensitization of PA smooth muscle. Constriction of PAs is associated with diminished reperfusion, leading us to hypothesize that increased small vessel resistance during I/R is an important contributor to incomplete reperfusion and expansion of infarct. Thus, Aim 1 is to investigate mechanisms of PA vasoconstriction during I/R and its relationship to infarct expansion. We will determine the relationship between PA vasoconstriction and perfusion deficit during I/R, and investigate mechanisms by which I/R promote smooth muscle calcium sensitization, including oxidative stress activation of Rho A kinase and protein kinase C. Our preliminary studies using spontaneously hypertensive stroke prone rats (SHRSP) found that PAs have increased tone prior to and after stroke that is associated with impaired endothelial potassium channel function, including small- and intermediate-conductance calcium-activated (SKCa/IKCa) and inward rectifier (Kir) potassium channels that may be central to vasodilation and increasing reperfusion blood flow. Thus, Aim 2 is to investigate mechanisms by which hypertension enhances PA vasoconstriction and perfusion deficit. We will determine the relationship between PA tone, smooth muscle calcium and membrane depolarization during hypertension and the role of angiotensin II in increasing L-type calcium channel activity and endothelin-1 production. We will also investigate mechanisms of potassium channel dysfunction in PAs during hypertension and their role in impaired vasodilation and infarction. The proposed studies will provide critically needed information on PA dysfunction during I/R and hypertension that is likely to be vital in relation to development of new but effective stroke therapy.